Leakage prevention between fluid flowing through the tubes and fluid flowing on the shell-side of an heat exchanger is desired to avoid violent chemical reactions that may occur if certain fluids involved in heat exchange were mixed with each other, and to avoid contamination of one fluid with the other to thereby simplify chemistry control of the fluids involved. For example, it is desirable to prevent contamination of water by either lithium in a fusion reactor steam generator or by seawater in the condenser of a conventional power plant.
In fusion power applications, there is the added requirement of avoiding the diffusion of tritium contained in liquid metal flowing on the shell-side of a steam generator through the tube walls to contaminate steam being generated in the tubes.
Tightly fitting double-walled tubes which are fabricated together for installation in an heat exchanger as a single piece are difficult to install. This is because the weld location for attaching the outer tube wall to its respective tube sheet is virtually inaccessible unless a large tube spacing is utilized along with special welding procedures that tend to be incompatible with reasonable shop costs. As a result, the pitch to diameter ratios of the outer tube sheet apertures of such constructions usually exceed 2.0 as compared to a typical ratio of 1.45 for single-walled tube heat exchangers. However, this higher ratio requires larger sized units and greater liquid metal inventory per thermal megawatt of heat exchange capacity, and results in lower shell-side heat transfer co-efficients than would be the case if the ratio could be reduced substantially. In addition to the resulting higher cost per square foot of heating surface, difficulties in locating and repairing leaks are experienced with such constructions.